Our broad objective is to understand the means by which Borrelia burgdorferi establishes an infection:transmission cycle between the tick vector and mammalian reservoir host, both of which are needed to maintain the spirochete in nature. The following research efforts relate to this goal. 1. Outer surface protein variation. The outer membrane of B. burgdorferi contains several abundant proteins (Osps) that vary in size and expression and are of unknown function. It is likely that the different Osps confer distinct properties on the spirochete that are pertinent to the different environments in which it must survive. Dr. Margolis cloned and sequenced an ospC gene that is variably expressed in culture and homologous to members of the large variable membrane protein family in B. hermsii. Analysis of the promoter region identified potential secondary structure in the RNA or DNA that could influence gene expression. Upstream of ospC are sequences homologous to 2 purine biosynthesis enzymes; the plasmid location of these gua genes is unique to Borrelia and may reflect an adaptation to disparate purine levels in ticks versus mammals. Dr. Tilly has demonstrated that one of these genes encodes a functional product by complementing an E. coli mutant deficient in this enzyme. Overlapping divergent promoters could permit co- regulated expression of the ospC and gua genes. 2. Plasmid structure and replication. Understanding the structure and replication of the unusual linear and circular Borrelia genome is of intrinsic interest and practical merit. Dr. Tilly has assayed Borrelia extracts for proteins that bind to telomeric sequences. She has isolated and sequenced the gene for the Borrelia HU homolog, a gene that may be involved in linear plasmid structure and replication. She has determined the genetic organization of the region, which includes two unknown open reading frames and homologs of the rho and rpsT genes and complemented E. coli mutants defective in some homologous genes. Hogan and Rosa have used a library constructed by Fuhrman to walk upstream from the ospAB operon toward the telomere of the 49-kb linear plasmid, which is predicted to have different sequence from those previously cloned. Hogan has identified evidence for an unusual integration event of sequences from one circular plasmid into another.